Many computer users and other entities have systems that utilize some form of security. Therefore, there often arises a need to prevent all but selected authorized persons from being able to carry out some defined transaction (such as granting of credit) or to gain access to electronic equipment or other system, facility or data. Preventing unauthorized clearance or access typically involves devices which limit access to the subject data, facility, or transaction to those who possess a unique physical device, such as a key or who know a fixed or predictable (hereinafter “fixed”) secret code. In at least some cases, relying on a fixed code or unique physical device as the means to control such selective clearance or access can mean that would-be unauthorized users need only obtain possession of the fixed code or unique device to gain such clearance or access. Typical instances of fixed codes include card numbers, user numbers or passwords issued to customers of computer data retrieval services.